Method of recovering ammonia gas from aqueous solution containing ammonia and carbon dioxide

ABSTRACT

A process for recovering ammonia gas from an aqueous solution containing ammonia and carbon dioxide which comprises countercurrently contacting such aqueous solution with a desorption gas selected from air, oxygen, nitrogen and mixtures thereof. The molar ratio of ammonia to carbon dioxide in the aqueous solution is at least 2, such aqueous solution being one obtained upon the synthesis of urea from ammonia and carbon dioxide or from the production of an aromatic nitrile by the ammoxidation of an aromatic hydrocarbon.

United States Patent 1 1111 3,594,937

[72] inventors Kenzo Cd: [56] References Cited W UNITED STATES PATENTSTakashi Ohara, Akashi-shi; Kazuhide Sato,

5 3 3333 Tamil/o9 313333332 32323 52132222 3 3i i: '11: 323/352 9 s211222 [22] Film 3 287 407 11 1966 2 fi 260/555 paemed 3 112 177 11/1963Ft gis e et ai 23/193 x [73] Ass'gms fgf 31251233 6/1966 Kunze et al.260/4653 1 1 Japan Gasoline Co" Lu. 3,262,962 7/1966 McDan el et al.260/465 .3 Tokyo Japan 3,282,860 11/1966 McDamelet al. 260/4653 [32]Priority Sept. 27, 1963 Primary Examiner-Joseph Paul Brust J lAttorneyNathaniel L. Leek [31] 38/514186 ABSTRACT: A process forrecovering ammonia gas from an [54] METHOD OF COVERING AMMONIA GASaqueous solution containing ammonia and carbon dioxide FROM AQUEOUSSOLUTION CONTAINING which comprises countercurrently contactin such aueous AMMONIA AND CARBON nroxmrz l h d g q 2 Claims 1 Drawing g t:monW1; at esorgltlon fga tIs hselcclted fzomfalr, oxygein, n1 ogen an mixures ereo e mo ar ra 1o 0 ammoma o [52] US. Cl 55/53, carbon dioxide inthe aqueous solution is at least 2, such aque- 55/70, 260/465 C, 260/555ous solution being one obtained upon the synthesis of urea [5 1] Int. Cl801d 19/00 from ammonia and carbon dioxide or from the production ofField of Search 260/4653, an aromatic nitrile by the ammoxidation of anaromatic 465 C 555; 23/193; /53, 68, hydrocarbon.

METHOD OF RECOVERING AMMONIA GAS FROM AQUEOUS SOLUTION CONTAININGAMMONIA AND CARBON DIOXIDE This invention relates to a method ofrecovering ammonia gas selectively from an aqueous solution containingammonia and carbon dioxide and, more particularly, to a methodofrecovering ammonia gas which is substantially free from carbon dioxideby contacting anaquequs solution containing ammonia'and carbon dioxidein which a molar ratio of ammonia to carbon dioxide is more than twowith a desorption gas, such as air or nitrogen which does not react withammonia and carbon dioxide and is substantially water-insoluble, at atemperature below about 60 C. and above the freezing pointof saidaqueous solution, usually in the range of about to 60 C., and thereafterdesorbing ammonia gas selectively from said aqueous solution.

In the synthesis of urea from ammonia and carbon dioxide or that ofnitriles by oxidation ofhydrocarbons with molecular oxygen in thepresence of ammonia (ammoxidation), a gaseous mixture containing greatquantities of carbon dioxide and.

unreacted or by-produced ammonia is formed. In order to. carry out suchsynthesis with commercial advantage, therefore, it is of paramountimportance to recover thesecomponentsand utilize them effectively.

Various techniques suitable for the above purposes have beenproposedupgto date as a method of recovering ammonia and carbon dioxide,in combination or separately, fromsuch gaseous mixture.

I One of such methods comprises scrubbing a gaseous mixture containingammonia and carbon dioxide with water to get.

ammonia and carbon dioxide directly. with an aqueous solu-' tion of a C0fixing agent such as organicand inorganic base or basic salt to removeselectively carbon dioxide or other acidic gasesout of the gaseousmixture.

Sincean aqueous solution of a C0 fixing agent such'as or.--

ganic and inorganic base or basic salt is required in stoichiometricamount.in these methods, theconsumption these chemical. agentsconstitutes an unavoidable setback. Furthermore it sometimes involvesthe corrosion -of an apparatus used.

An object of this-invention is to provide an improved andeconomicalmethod of recovering, ammonia-gas whichis substantially free. fromcarbon dioxide, overcoming technological.

drawbacks of the conventional methods, when ammonia gas is.

to be selectively recovered from an aqueous solution containing ammoniaand carbon dioxide.

Another object of this invention is to provide amethod ofrecovering-ammonia gas which substantially free from carbon dioxidefroman aqueous solution containing-ammonia-and carbon dioxide in which amolar ratio ofiammoniato carbon dioxide is more than 2', which solutionis obtained by scrubbing with water a gaseous mixture containing ammoniaand carbon dioxide which is formed in the .synthesis of.urea or themanufacture of nitriles from hydrocarbons by ammoxidation. In theproduction of nitriles, especially aromatic nitriles from thecorresponding hydrocarbons by ammoxidation, for instance in theproduction of benzonitrile from tolueneor that of phthalonitriles fromxylenes, a great quantity of unreacted' ammonia is present in theproduct gas along with carbon dioxide.

It is especially important in the ammoxidation ofiaromatic hydrocarbonsto recover ammonia is such manner that it may be most advantageouslyrecycled to the reaction system.

According to this invention it is possible to desorb ammonia free fromcarbon dioxide from an aqueous solution containing NH; and CO with theaid of air and/or oxygen as a desorption gas. The resulting-gaseousmixture containing ammonia can be directly recycled to the ammoxidationreactor with further addition of feedstock hydrocarbonand fresh ammonia.Therefore; the method of this invention is particularly advantageouswhen applied to the manufacture of nitriles by oxidation of hydrocarbonswith molecular oxygen in the presence of ammonia. It is of courseapplicable to the ammoxidation of aliphatic hydrocarbons.

In general, when gas is to be absorbed in a liquid medium, it is ageneralrule to select relatively low temperatures and high pressures asthe operational condition'l Thea'mount of gas absorbed is maximizedunder that condition. On the other hand, when gas is to desorbedfromasolution which has absorbed the gas, the choice of high temperaturesand low pressures is preferred.

However, when ammonia is desorbed at high'temperatures from an aqueoussolution containing ammonia and carbon dioxide in which a molar ratio ofammonia to carbonadioxide is more than two, carbon dioxide as well asammonia is desorbed by the decomposition of a compoundconsistingofammonia and carbon dioxide.

It is already known that in an aqueous solution in which ammonia andcarbon-dioxide are dissolved together, the partial pressures of ammoniaand carbon dioxide in a gaseous phase which is present inequilibrifiiii/with the aqueous solution vary depending upon the change6? temperature and molar ratio of ammonia to carbon dioxide. As anexample, Tables 1 and 2 respectively showpartial pressures atequilibrium of ammonia and carbon dioxide according to variations of amolar ratio of ammonia to carbon dioxide atconstant temperature andthose according to thechange of temperature'with a molar ratio ofammonia to carbon dioxide maintained constant both in respect of anaqueous solution in which the concentration-of ammonia is 2 perlitre.

Table l (Temperature 20 C.)

With the knowledge of the equilibrium, we have found thatv itis-possible to recover ammonia gas substantially free from carbondioxide, without the necessity of adding a C0 fixing agent asmentionedabove, by contacting a desorption gas which does not react withammonia and carbon dioxide and is substantially water-insoluble with anaqueous solution containing NH; and'CO, in which a molar ratio ofammonia to carbon dioxide is more than two, at a temperature in therange where the decomposition of the compound consisting of ammonia andcarbon dioxide is not appreciable, i.e., below 60 C. and above thefreezing point of the aqueous solution.

Hence, this invention relates to the method ofselectively recoveringammonia which comprises desorbing at relatively low temperatures ammoniawhich is substantially free from carbon dioxide from an aqueous solutioncontaining ammonia and carbon dioxide in which a molar ratio of ammoniato carbon dioxide is more than two.

Such aqueous solution may contain a solvent such as methanol. It is tobe understood, therefore, that the term aqueous solution containingammonia and carbon dioxide" used in this invention is a generic tenn fora solution of an aqueous medium capable of absorbing said gaseousmixture.

The molar ratio of ammonia to carbon dioxide should be more than 2, andthere is no upper limit.

Other objects and advantages of this invention will become more apparentfrom the following description.

In accordance with the method of this invention, the feed aqueoussolution is fully contacted with a desorption gas which does not reactwith ammonia and carbon dioxide and is substantiallv water-insoluble,and thereafter ammonia gas is selectively desorbed from said aqueoussolution. In this method, the temperature at which the solution issubjected to said gas is an important requirement inseparable from thesaid molar ratio. This temperature is below about 60 C. and above thefreezing point of said aqueous solution, but in actual practice atemperature usually in the range of an ambient temperature to 60 C., andin many cases to 60 C., is employed. When the temperature exceeds about60 C., carbon dioxide comes into the desorbed gas. It is necessarytherefore to adjust the operation so that the temperature may not exceedabout 60 C. So long as the method is practiced at a temperature in theabove-mentioned range, ammonia gas alone is selectively recoveredtogether with the desorption gas, contrary to the above-mentionedtechnological common knowledge about the desorption of gas.

The desorption gas should be inert to ammonia and carbon dioxide andsubstantially water-insoluble. As such desorption gas, there areadvantageously used air, oxygen, nitrogen and a mixture of more than twoof these gases.

In the method of this invention, an aqueous solution containing ammoniaand carbon dioxide in which a molar ratio of ammonia to carbon dioxideis more than two is fully contacted with said desorption gas at atemperature specified above by using any conventional means ofcontacting liquid with gas. As

such means, there are employed, for instance, a counter current typemultiplate column system, a counter current type from which a part ofammonia has been desorbed comes down onto one plate after another, andwhile desorbing the dissolved ammonia further, finally reaches the plateat the bottom. In this operation, bubble cap plates may be used. It ispreferable that roughly all of the ammonia which is dissolved in thesaid solution in excess of carbon dioxide is desorbed but, in actualpractice, an adjustment is so made that there may be a small amount ofresidual ammonia in the solution. The aqueous solution from which afixed amount of ammonia has been desorbed is taken out from a pipe 4 asan aqueous solution at the bottom of the column. The desorption gas isintroduced from a pipe 5 located at the bottom of the column.

The aqueous solution is deprived of heat of vaporization while itdischarges ammonia in the course of its descending in the desorptioncolumn. This results in the lowering of temperature. As the lowering oftemperature decreases a desorption effect, it is preferable to takewhole or part of the solution on the plate out of the desorption columnas occasion demands, to raise its temperature to a desired point belowabout C. in a heater 2, to return it into the desorption column, andthereby to maintain the interior of the desorption column at atemperature in the specified range suitable for operation. The heater isprovided at an appropriate position in the desorption column. Heatingmay be effected with the provision of an interior heater in thedesorption column. Furthermore, not only a solution containing ammoniaand carbon dioxide but also the desorption gas may be introduced afterpreheated to a desired temperature below about 60 C. The operation isperformed usually without employing any means for elevating or reducingthe pressure, but, if desired, it may be carried out under an elevatedor reduced pressure.

This invention is further explained by the examples which follow.

EXAMPLE 1 An aqueous solution containing 24.5 mole percent of ammoniaand 2.0 mole percent of carbon dioxide was fed into the uppermost plateof said desorption column and was contacted in counter current with airintroduced from the bottom part of the column, thereby desorbingammonia. Results are shown in Table 3.

TABLE 3 Feed aqueous Aqueous solution at Solution Air Recovered gasbottom of column M01 01 M01 M01 KgJhr. percent Kg./hr. percent KgJhr.percent KgJhr. percent NHa 292. 5 2-1. 5 224. 5 13. 87 68.0 7. 0

62. 5 2. 0 0. 4-1 0. 01 62. 06 2. 92870 73. 5 34. 2 2. 3 36.0 2. 10 926.2 90.5 Air 2, 320. 0 97. 7 2, 320. 0 84. 02

Total 1, 283.0 100.0 2, 354. 2 100.0 2, 580. 9-1 100.00 1,056. 26 100.0

pipe system and an agitation mixing system.

Now, with reference to the accompanying drawing, an example of operationby means of a counter current type multiplate column is explained indetail.

The drawing shows an example of the apparatus to be used in the practiceof this invention.

An aqueous solution containing ammonia and carbon dioxide in which amolar ratio of ammonia to carbonic dioxide is at least two is fed intothe uppermost plate 3 of a desorption column 1 and is contacted with adesorption gas which ascends from the bottom, thereby desorbing part ofammonia dissolved in the aqueous solution. The said aqueous solution Thedesorption column was maintained at a pressure of 1.6 atmospheres(absolute pressure) and at a temperature in the range of 20 to 40 C.

EXAMPLE 2 TABLE 4 Feed aqueous Aqueous solution at solution AirRecovered gas bottom of column Kg./hr. percent Kg./hr. percent Kg./hr.percent Kg. lhr. percent NHs 85.0 8. 6 '25. 5 12. 82 59. 5 6.2 CO: 62. 52.5 0. 01 0.01 62. 46 2. 5 H1O 928. 0 S8. 9 3.6 2. 0 3. 6 1. 71 928.091. 3 Air 290. 0 98 0 290. 0 85. -16

Total. 1, 075. 5 100.0 293. 6 100. 0 319. 14 1.00. 00 1, 049. 96 100. 0

The desorption column was maintained at a pressure of 1.6 atmospheres(absolute pressure) and a temperature in the range of to 40 C.

In accordance with this invention, as is clear from the foregoingExamples, the content of ammonia in the recovered gas is exceedinglygreat as compared with the carbon dioxide, It is possible, therefore, torecover ammonia substantially free from carbon dioxide.

In the foregoing Examples air was used as a desorption gas.

The use of air and/or oxygen gives an advantage that a gaseous mixtureof recovered ammonia gas and a desorption gas can be used directly asthe feed gas for ammoxidation. The method of this invention can beadvantageously practiced in combination with a process which forms anaqueous solution containing ammonia and carbon dioxide in which a molarratio of ammonia to carbon dioxide is more than two or with a process toabsorb ammonia preferentially from a gaseous mixture which is generatedby .heating and decomposing the aqueous solution in which the molarratio is less than 2. For instance, as the solution at the bottom of thecolumn from which ammonia has been desorbed still contains aconsiderable amount of ammonia, it is possible to further recoverammonia by combining a method to absorb ammonia preferentially from agaseous mixture generated upon heating the said aqueous solution at thebottom of the column. The liquid at the bottom of the column which isdischarged from the'desorption column may be used again as agas-absorbing solvent directly or in dilution with water We claim:

, l. A method of recovering ammonia gas from an aqueous solutioncontaining ammonia and carbon dioxide in a molar ration of ammonia tocarbon dioxide of from 2:1 to 12.2511, said aqueous solution beingobtained from the production of aromatic nitriles by the ammoxidation ofaromatic hydrocarbons, which method comprises concurrently contactingsaid aqueous solution with a desorption gas selected from the groupconsisting of air, oxygen, nitrogen and mixtures thereof, at atemperature below about 66 C. and above the ffeeiirig point of saidaqueous solution, thereby desorbing from said aqueous solution ammoniagas which is substantially free of carbon dioxide.

2. A method of recovering ammonia gas from an aqueous solutioncontaining ammonia and carbon dioxide in a molar ratio of ammonia tocarbon dioxide of from 2:1 to 12.25:], said aqueous solution beingobtained upon synthesis of urea from ammonia and carbon dioxide, whichmethod comprises concurrently contacting said aqueous solution with adesorption gas selected from the group consisting of air, oxygen,nitrogen and mixtures thereof, at a temperature below about 60 C. andabove the freezing point of said aqueous solution, thereby desorbingfrom said aqueous solution ammonia gas which is substantially free ofcarbon dioxide.

2. A method of recovering ammonia gas from an aqueous solutioncontaining ammonia and carbon dioxide in a molar ratio of ammonia tocarbon dioxide of from 2:1 to 12.25:1, said aqueous solution beingobtained upon synthesis of urea from ammonia and carbon dioxide, whichmethod comprises concurrently contacting said aqueous solution with adesorption gas selected from the group consisting of air, oxygen,nitrogen and mixtures thereof, at a temperature below about 60* C. andabove the freezing point of said aqueous solution, thereby desorbingfrom said aqueous solution ammonia gas which is substantially free ofcarbon dioxide.